Optical waveguides for use in electro-optic modulators and switches prepared by diffusing niobium into a lithium tantalate crystal have been disclosed in my copending application "Novel Optical Waveguides" Ser. No. 555,725 filed Mar. 6, 1975, which is a continuation-in-part of application Ser. No. 442,844 filed Feb. 15, 1974, now abandoned, which is a continuation-in-part of application Ser. No. 434,408 filed Jan. 18, 1974, now abandoned. These waveguides are simple to make and they can be employed to make electro-optic devices which have high efficiency but low power and voltage requirements.
In a preferred embodiment for preparing these waveguides, a polished crystal of lithium tantalate is coated with a thin (400-1,000 angstroms) layer of niobium and heated to a temperature in the range of about 1,050.degree. to 1,250.degree. C. in the presence of oxygen to form niobium oxide and to allow the niobium to diffuse into the crystal. The resultant crystal has a surface layer of the formula LiNb.sub.x Ta.sub.1.sub.-x O.sub.3. The niobium diffuses into the layer to a distance which can be controlled by employing proper temperatures, annealing time and the like. However, since lithium niobate and lithium tantalate have the same crystal structure, generally a solid solution of these two crystals is obtained forming a good single crystal waveguide.
One disadvantage to the above-described waveguides is the formation of defect areas and a rough surface on the crystals. It is believed these imperfections are formed due to the occasional formation of precipitates or polycrystalline phases in the crystal due to the excess of niobium near the surface of the crystal. Such defects result in increased scattering of light in the waveguide and attenuation of light propagating in the waveguide, with concomitant losses in efficiency. A method of decreasing such losses has been sought.